Security expert Michael Messner has identified several holes in D-Link's DIR-300 and DIR-600 routers that allow potential attackers to execute arbitrary commands with little effort. Although current firmware versions are also affected, the router manufacturer does not appear to be planning to close the hole.

Messner describes on his blog how a simple POST parameter allows Linux commands to be executed at root level on vulnerable routers. No password or other authentication is required to do so. In a short test, The H's associates at heise Security found that many of the devices can even be accessed from the internet and managed to inject a harmless command into such a router. A real attacker could randomly exploit systems, for example to divert a router's entire internet traffic to a third-party server.

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You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump